In 2016, the next one will be appointed...

The current list of official candidates can be found on the President of the UN General Assembly's website

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The General Assembly will, "without prejudice to the role of the principle organs as enshrined in Article 97 of the Charter, conduct informal dialogues or meetings with candidates for the position of Secretary-General, without any prejudice to any candidate who does not participate, thus contributing to the transparency and inclusivity of the process."